1. Field of the invention
The present invention relates to a radio frequency identification (RFID) tag and an RFID system having the same. More particularly, the present invention relates to an RFID tag to stably control impedance on transmitting a carrier wave and an RFID system having the same.
2. Description of the Related Art
RFID is an automatic recognition technology using wireless frequencies and is a representative new technology of a contactless integrated circuit (IC) card to replace a barcode and a magnetic card.
The RFID system includes an RFID reader, a host computer and a transponder, that is, an RFID tag.
The RFID reader transmits radio waves to the RFID tag, and the RFID tag receives the radio waves and transmits corresponding data to the RFID reader.
The RFID tag includes an antenna transmitting and receiving radio waves to/from the RFID reader and a driving chip storing data such as identification information to identify each RFID tag. If the RFID tag receives radio waves from the RFID reader, the RFID tag transmits the corresponding data including the identification information to the RFID reader.
The RFID tag separates into active and passive tags according to the operation method. The active RFID tag has a power source to drive itself. Meanwhile, the passive RFID tag does not have a power source to drive itself, but receives radio waves and a power to drive itself from the RFID reader.
After receiving an input power, the passive RFID tag generates a carrier wave and transmits a carrier power to the RFID reader. The RFID tag loads predetermined data including the identification information into the carrier wave, modulates the data into electric signals and transmits the power to the RFID reader.
The RFID tag adjusts a resistance corresponding to the driving chip by turning on/off a transistor embedded in the driving chip on transmitting the carrier wave, so that impedance of the driving chip is adjusted. Impedance adjustment of the RFID tag is essential to minimize reflection signals between the RFID reader and the RFID tag. That is, if impedance matching between the RFID reader and the RFID tag is not accurately performed, a reflection coefficient between the RFID reader and the RFID tag become high and the reflection signals increase so that power loss of the RFID reader increases.
To prevent this, the RFID tag adjusts resistance of the driving chip according to the size of the carrier wave for impedance matching using a modulation transistor.
FIG. 1 is a graph illustrating an output voltage of the RFID tag corresponding to signal transmission and reception in a related art RFID system.
Referring to FIG. 1, the RFID reader transmits an input power (IP) and the RFID tag receives the IP. The RFID tag transmits a reflection power (RP) to transmit a carrier wave corresponding to the IP to the RFID reader. The RP contains a carrier wave modulated to include a signal wave indicating the identification information of the RFID tag.
The amplitude of the carrier wave varies corresponding to the amplitude of the signal wave, that is, a digital data value of the signal wave in the modulation process. That is, the driving chip of the RFID tag changes the amplitude of the carrier wave by adjusting the voltage. Accordingly, the impedance of the driving chip can vary corresponding to the amplitude variation of the data signal.
To prevent this, the RFID tag modifies a resistance of the driving chip using the modulation transistor. That is, the modulation transistor is turned on/off corresponding to the amplitude variation of the carrier wave to adjust the current of the driving chip. Accordingly, the RFID tag can adjust the resistance of the driving chip so that the impedance of the driving chip can be adjusted.
However, if the modulation transistor is turned on so that current consumption increases, the intensity of voltage transmitted from the driving chip can be lower than the reference voltage Vdd (A, B). The voltage output from the RFID tag, that is, the voltage transmitted from the driving chip has to be higher than the reference voltage Vdd. If the modulation transistor is turned off again, the current consumption is suddenly reduced. Accordingly, the voltage output from the driving chip is higher than the reference voltage Vdd.
As described above, as the variation of voltage transmitted from the driving chip increases according to the amplitude variation of a data signal in the RFID tag, circuits embedded in the RFID tag can unstably drive. Accordingly, the power supply of the RFID tag is unstable, and modulation and demodulation are unstable, so that signal transmission/reception between the RFID reader and the RFID tag cannot be normally performed.